Pentagon officials said Hagel, who was attending a NATO ministerial meeting in Brussels, had not yet been successful in trying to contact his Russian counterpart, Sergei Shoygu, to urge restraint despite Russia's stated policy of non-intervention.

The Pentagon officials stressed that no alerts had gone out to U.S. forces in Europe.

Also attending the Brussels meeting was Ukraine's ambassador to NATO, Ihor Dolhov. He charged that the Russian military maneuvers were aimed at unsettling Ukraine's new government.

Dolhov called on Russia to "refrain from actions leading to destabilization of Ukraine, particularly in Crimea" where there were reports that Russian armored vehicles had been seen on the streets of Sevastopol outside the confines of the leased naval base for the Russian Black Sea fleet.

In Moscow, Russian Deputy Defense Minister Anatoly Antonov said the exercises in Russia's western military district near the border with Ukraine followed an alert notice from Putin.

Antonov said the "snap exercises" called to gauge readiness involved 150,000 troops from all services, 90 aircraft, 120 helicopters and 880 military vehicles, the Russian news agency RIA Novosti reported.

Putin has yet to speak publicly on the tumultuous events in Ukraine that led to bloody street demonstrations and the ouster of Moscow-friendly President Victor Yanukovych.

The unrest also sparked fears in Kiev, the seat of Ukraine's parliament, Putin might seek to foment a breakaway movement in Russian-speaking eastern Ukraine.

In Moscow on Wednesday, Foreign Minister Sergei Lavrov denied Ukraine's allegations of Russian meddling in eastern Ukraine and also warned the U.S. and Europe to avoid involvement.

"We have confirmed our principled position to not interfere in Ukraine's internal affairs and expect all (foreign powers) to follow a similar logic," Lavrov said.

Earlier this week, Secretary of State John Kerry warned that Russia would be making a "grave mistake" by interfering in Ukraine but he also played down the possibility of a Cold War-type confrontation between the U.S. and Russia.

"This is not ‘Rocky IV,'" Kerry said, a reference to the movie in which the Rocky Balboa character fights a Russian.